A key number of Republicans have warned US President over his increasing belligerence toward special counsel Robert Mueller.

US House Speaker Paul Ryan and a number of other key Republicans on Sunday have warned US President Donald Trump over his increasing belligerence toward special counsel Robert Mueller and his ongoing Russia investigation.

US House Speaker said that President Trump should let special counsel Robert Mueller carry out his investigation into alleged Russian meddling in the 2016 election.

Senator Lindsey Graham of South Carolina also warned Trump over the matter, saying that if Trump tried to fire Mueller, “that would be the beginning of the end of his presidency.”

Republican Senator Jeff Flake of Arizona told CNN’s “State of the Union” on Sunday that he thinks Trump is moving closer to firing Mueller and that he expects Republicans to push back against any effort by Trump to end the investigation.

Following the abrupt firing of FBI Deputy Director Andrew McCabe by Attorney General Jeff Sessions  on Friday, Trump has ratcheted up his criticism against the ongoing investigation about Russian meddling in the 2016 presidential election.

Trump asserted on Saturday that Mueller's investigation into possible collusion between the Trump campaign and Russia should never have been opened in the first place.

Mueller, a Republican, has obtained guilty pleas from three former Trump associates, and indicted former Trump campaign chairman Paul Manafort.